Hello everyone,
Due to the confusion that seems to about the horribly complex
Linux/i386 boot protocol, I have written up the following document. I
would like to get feedback both on the writing and the technical
contents before sending it to Linus.
Thanks!
-hpa
THE LINUX/I386 BOOT PROTOCOL
----------------------------
Updated as of protocol version 2.02
On the i386 platform, the Linux kernel uses a rather complicated boot
convention. This has evolved partially due to historical aspects, as
well as the desire in the early days to have the kernel itself be a
bootable image, the complicated PC memory model and due to changed
expectations in the PC industry caused by the effective demise of
real-mode DOS as a mainstream operating system.
Currently, four versions of the Linux/i386 boot protocol exist.
Old kernels: zImage/Image support only. Some very early kernels
may not even support a command line.
Protocol 2.00: Added bzImage and initrd support, as well as a
formalized way to communicate between the boot loader
and the kernel.
Protocol 2.01: Added a heap overrun warning.
Protocol 2.02: New command line protocol. Lower the
conventional-memory ceiling. zImage deprecated but
still supported.
**** MEMORY LAYOUT
The traditional memory map for the kernel loader, used for Image or
zImage kernels, typically looks like:
| |
0A0000 +------------------------+
| Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
09A000 +------------------------+
| Stack/heap/cmdline | For use by the kernel real-mode code.
098000 +------------------------+
| Kernel setup | The kernel real-mode code.
090200 +------------------------+
| Kernel boot sector | The kernel legacy boot sector.
090000 +------------------------+
| Protected-mode kernel | The bulk of the kernel image.
010000 +------------------------+
| Boot loader | <- Boot sector entry point 0000:7C00
001000 +------------------------+
| Reserved for MBR/BIOS |
000800 +------------------------+
| Typically used by MBR |
000600 +------------------------+
| BIOS use only |
000000 +------------------------+
When using bzImage, the protected-mode kernel was relocated to
0x100000 ("high memory"), and the kernel real-mode block (boot sector,
setup, and stack/heap) was made relocatable to any address starting at
0x10000. Unfortunately, in protocols 2.00 and 2.01 the command line
is still required to live in the 0x9XXXX memory range, and that memory
range is still overwritten by the early kernel. The 2.02 protocol
fixes that.
It is desirable to keep the "memory ceiling" -- the highest point in
low memory touched by the boot loader -- as low as possible, since
some newer BIOSes have begun to allocate some rather large amounts of
memory near the top of low memory. The boot loader should use the
"INT 12h" BIOS call to verify how much low memory is available.
**** THE REAL-MODE KERNEL HEADER
In the following text, and anywhere in the kernel boot sequence, "a
sector" refers to 512 bytes. It is independent of the actual sector
size of the underlying medium.
The first step in loading a Linux kernel should be to load the
real-mode code (boot sector and setup code) and then examine the
following header at offset 0x01f1. The real-mode code can total up to
32K, although the boot loader may choose to load only the first two
sectors (1K) and then examine the bootup sector size.
The header looks like:
Offset Proto Name Meaning
Size
01F1/1 ALL setup_sects The size of the setup in sectors
01F2/2 ALL root_flags If set, the root is mounted readonly
01F4/2 ALL syssize DO NOT USE - for bootsect.S use only
01F6/2 ALL swap_dev DO NOT USE - obsolete
01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
01FA/2 ALL vid_mode Video mode control
01FC/2 ALL root_dev Default root device number
01FE/2 ALL boot_flag 0xAA55 magic number
0200/2 2.00+ jump Jump instruction
0202/4 2.00+ header Magic signature "HdrS"
0206/2 2.00+ version Boot protocol version supported
0208/4 2.00+ realmode_swtch Boot loader hook (see below)
020C/4 2.00+ start_sys Points to kernel version string
0210/1 2.00+ type_of_loader Boot loader identifier
0211/1 2.00+ loadflags Boot protocol option flags
0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
0214/4 2.00+ code32_start Boot loader hook (see below)
0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
0224/4 2.01+ heap_end_ptr Free memory after setup end
0226/2 N/A pad1 Unused
0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
If the setup_sects field contains zero (0), the real value is 4.
If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
the boot protocol version is "old". Loading an old kernel, the
following parameters should be assumed:
setup_sects = 4
Image type = zImage
initrd not supported
Real-mode kernel must be located at 0x90000.
Otherwise, the "version" field contains the protocol version,
e.g. protocol version 2.01 will contain 0x0201 in this field. When
setting fields in the header, you must make sure only to set fields
supported by the protocol version in use.
Most boot loaders will simply load the kernel at its target address
directly. Such a boot loader do not need to worry about filling in
most of the fields in the header. The following fields should be
filled out, however:
type_of_loader:
If your boot loader has an identifier assigned in
arch/i386/boot/setup.S, enter that value. Otherwise, enter
0xFF here.
loadflags, heap_end_ptr:
If the protocol version is 2.01 or higher, enter the
offset limit of the setup heap into heap_end_ptr and set the
0x80 bit (CAN_USE_HEAP) of loadflags. heap_end_ptr appears to
be relative to the start of setup (offset 0x0200).
setup_move_size:
When using protocol 2.00 or 2.01, if the real mode
kernel is not loaded at 0x90000, it gets moved there later in
the loading sequence. Fill in this field if you want
additional data (such as the kernel command line) moved in
addition to the real-mode kernel itself.
ramdisk_image, ramdisk_size:
If your boot loader has loaded an initial ramdisk (initrd),
set ramdisk_image to the 32-bit pointer to the ramdisk data
and the ramdisk_size to the size of the ramdisk data.
The initrd should typically be located as high in memory as
possible, as it may otherwise get overwritten by the early
kernel initialization sequence. However, it must never be
located above address 0x3C000000 if you want all kernels to
read it.
cmd_line_ptr:
If the protocol version is 2.02 or higher, this is a 32-bit
pointer to the kernel command line. The kernel command line
can be located anywhere between the end of setup and 0xA0000.
Fill in this field even if your boot loader does not support a
command line, in which case you can point this to an empty
string (or better yet, to the string "auto".) If this field
is left at zero, the kernel will assume that your boot loader
does not support the 2.02 protocol.
**** THE KERNEL COMMAND LINE
The kernel command line has become an important way for the boot
loader to communicate with the kernel. Some of its options are also
relevant to the boot loader itself, see "special command line options"
below.
The kernel command line is a null-terminated string up to 255
characters long, plus the final null.
If the boot protocol version is 2.02 or later, the address of the
kernel command line is given by the header field cmd_line_ptr (see
above.)
If the protocol version is *not* 2.02 or higher, the kernel
command line is entered using the following protocol:
At offset 0020 (word), enter the magic number 0xA33F.
At offset 0022 (word), enter the offset of the kernel
command line (relative to the start of the real-mode kernel).
The kernel command line *must* be within the memory region
covered by setup_move_size, so you may need to adjust this
field.
**** SAMPLE BOOT CONFIGURATION
As a sample configuration, assume the following layout of the real
mode segment:
0x0000-0x7FFF Real mode kernel
0x8000-0x8FFF Stack and heap
0x9000-0x90FF Kernel command line
Such a boot loader should enter the following fields in the header:
unsigned long base_ptr; /* base address for real-mode segment */
if ( setup_sects == 0 ) {
setup_sects = 4;
}
if ( protocol >= 0x0200 ) {
type_of_loader = <type code>;
if ( loading_initrd ) {
ramdisk_image = <initrd_address>;
ramdisk_size = <initrd_size>;
}
if ( protcol >= 0x0201 ) {
heap_end_ptr = 0x9000 - 0x200;
loadflags |= 0x80; /* CAN_USE_HEAP */
}
if ( protocol >= 0x0202 ) {
cmd_line_ptr = base_ptr + 0x9000;
} else {
cmd_line_magic = 0xA33F;
cmd_line_offset = 0x9000;
setup_move_size = 0x9100;
}
} else {
/* Very old kernel */
cmd_line_magic = 0xA33F;
cmd_line_offset = 0x9000;
/* A very old kernel MUST have its real-mode code
loaded at 0x90000 */
if ( base_ptr != 0x90000 ) {
/* Copy the real-mode kernel */
memcpy(0x90000, base_ptr, (setup_sects+1)*512);
/* Copy the command line */
memcpy(0x99000, base_ptr+0x9000, 256);
}
/* It is recommended to clear the following memory */
memset(0x90000 + (setup_sects-1)*512, 0,
(64-setup_sects-1)*512);
}
**** LOADING THE REST OF THE KERNEL
The non-real-mode kernel starts at offset (setup_sects+1)*512 in the
kernel file (again, if setup_sects == 0 the real value is 4.) It
should be loaded at address 0x10000 for Image/zImage kernels and
0x100000 for bzImage kernels.
The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
bit (LOAD_HIGH) in the loadflags field is set:
is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
load_address = is_bzImage ? 0x100000 : 0x10000;
Note that Image/zImage kernels can be up to 512K in size, and thus use
the entire 0x10000-0x90000 range of memory. This means it is pretty
much a requirement for these kernels to load the real-mode part at
0x90000. bzImage kernels allow much more flexibility.
**** SPECIAL COMMAND LINE OPTIONS
If the command line provided by the boot loader is entered by the
user, the user may expect the following command line options to work.
They should normally not be deleted from the kernel command line even
though not all of them are actually meningful to the kernel.
vga=<mode>
<mode> here is either an integer (in C notation, either
decimal, octal, or hexadecimal) or one of the strings
"normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
(meaning 0xFFFD). This value should be entered into the
vid_mode field, as it is used by the kernel before the command
line is parsed.
mem=<size>
<size> is an integer in C notation optionally followed by K, M
or G (meaning << 10, << 20 or << 30). This specifies to the
kernel the memory size. This affects the possible placement
of an initrd, since an initrd should be placed near end of
memory.
initrd=<file>
An initrd should be loaded. The meaning of <file> is
obviously bootloader-dependent.
In addition, some boot loaders add the following options to the
user-specified command line:
BOOT_IMAGE=<file>
The boot image which was loaded. Again, the meaning of <file>
is obviously bootloader-dependent.
auto
The kernel was booted without explicit user intervention.
If these options are added by the boot loader, it is highly
recommended that they are located *first*, before the user-specified
or configuration-specified command line. Otherwise, "init=/bin/sh"
gets confused by the "auto" option.
**** RUNNING THE KERNEL
The kernel is started by jumping to the kernel entry point, which is
located at *segment* offset 0x20 from the start of the real mode
kernel. This means that if you loaded your real-mode kernel code at
0x90000, the kernel entry point is 9020:0000.
At entry, ds = es = ss = fs = gs should point to the start of the
real-mode kernel code (0x9000 if the code is loaded at 0x90000), and
sp should be set up properly, normally pointing to the top of the
heap. In our example from above, we would do:
seg = base_ptr >> 4;
cli(); /* Enter with interrupts disabled! */
_SS = seg;
_SP = 0x9000; /* Load SP right after loading SS! */
_DS = _ES = _FS = _GS = seg;
jmp_far(seg+0x20, 0); /* Run the kernel */
If your boot sector accesses a floppy drive, it is recommended to
switch off the floppy motor before running the kernel, since the
kernel boot leaves interrupts off and thus the motor will not be
switched off.
**** ADVANCED BOOT TIME HOOKS
If the boot loader runs in a particularly hostile environment (such as
LOADLIN, which runs under DOS) it may be impossible to follow the
standard memory location requirements. Such a boot loader may use the
following hooks that, if set, are invoked by the kernel at the
appropriate time. The use of these hooks should probably be
considered an absolutely last resort!
IMPORTANT: All the hooks are required to preserve %ebp, %esi and %edi
across invocation.
realmode_swtch:
A 16-bit real mode far subroutine invoked immediately before
entering protected mode. The default routine disables NMI, so
your routine should probably do so, too.
code32_start:
A 32-bit flat-mode routine *jumped* to immediately after the
transition to protected mode, but before the kernel is
uncompressed. No segments, except CS, are set up; you should
set them up to KERNEL_DS (0x18) yourself.
After completing your hook, you should jump to the address
that was in this field before your boot loader overwrote it.
-- <hpa@transmeta.com> at work, <hpa@zytor.com> in private! "Unix gives you enough rope to shoot yourself in the foot." http://www.zytor.com/~hpa/puzzle.txt- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Fri Jul 14 2000 - 18:51:10 EDT